The present invention relates to xe2x80x9cactive implantable medical devicesxe2x80x9d as such devices are defined by the Jun. 20, 1990 directive 90/385/CEE of the Council of the European Cornmunities, more particularly to pacemaker, defibrillator and/or cardiovertor devices that are able to deliver to the heart stimulation pulses of low energy for the treatment of heartbeat rate disorders. The invention is more particularly directed to the prostheses known as xe2x80x9cmultisitexe2x80x9d, in which respective electrodes are placed in a plurality of distinct respective cardiac sites comprising at least one ventricular site and one atrial site. This prosthesis can be of the xe2x80x9cdouble chamberxe2x80x9d (right atrial stimulation and right ventricular stimulation) or, generally, xe2x80x9ctriple chamberxe2x80x9d (right atrial stimulation and double ventricular stimulation) or xe2x80x9cquadruple chamberxe2x80x9d (double atrial stimulation and double ventricular stimulation) type.
The control of stimulation implies making an adjustment of various control parameters, such as the stimulation frequency, the atrio-ventricular delay (AVD), or the interventricular delay in the case of a biventricular stimulation.
These various parameters are typically adjusted according to signals delivered by sensors, for example, a minute ventilation (MV) sensor. The minute ventilation is a factor which is representative of the instantaneous metabolic needs of the patient. This factor, in a known manner, is evaluated by measurement of the trans-pulmonary bio-impedance, i.e., between the heart and the case of pacemaker, where the case is located in the top of the thorax. This bio-impedance value is measured by injecting a current pulse between the case and a first cardiac electrode, and collecting (detecting) a signal that is differential potential in response to the current pulse between the case and a second point.
Another factor which is desirable to know is the cardiac flow. It can be interesting, particularly with a multisite pacemaker, to obtain an indication of this flow and thus of the fraction of ejection. The fraction of ejection is the hemodynamic reference parameter used to optimize stimulation on the various cardiac sites. This cardiac flow can be evaluated by measurement of the intracardiac pressure, for example, as proposed in the published application WO-A-99/34863 (Pacesetter AB), but at an expense of requiring a specific probe incorporating a piezoelectric sensor and particular associated electronics to condition the signals resulting from this sensor, to convert them and transmit them to the microprocessor of the pacemaker for processing and use.
Another parameter correlated with the cardiac flow is the transvalvular impedance, a parameter that is generally measured on the right heart, for example, as proposed in U.S. Pat. No. 5,154,171 (Chirife). This document proposes to take the bio-impedance measurement by injecting a current pulse between a ventricular site and an atrial site, and collecting a differential potential between these same two points. In practice, however, it is noted that this configuration (a bipolar configuration of two electrodes) of injection/collection appears sensitive to the movement of the probes containing the electrodes, and does not allow a reliable and precise measurement of the impedance. Moreover, this technique also requires use of a particular electronic circuit to inject the current and collect the signals in response, to convert the collected signals and transmit them for treatment by the microprocessor of the pacemaker.
U.S. Pat. No. 5,501,702 (Plicchi) proposes a configuration that uses the circuits for the measurement of the minute ventilation by intracardiac injection/collection also to perform the trans-pulmonary current injection/signal collection, so as to measure intracardiac bio-impedance values correlated with hemodynamic parameters such as cardiac flow and fraction of ejection. But this known device also uses complex and multiple switching, wherein a particular cathode and anode are connected to switching transistors to ensure the appropriate electric connections between the case and the measuring circuit, the atrial electrodes, and the ventricular electrodes. In fact, these multiple switches involve a system that is so complex a system that in practice it is unrealizable. This is particularly true in a multisite type of device.
It is, therefore, an object of the present invention to propose an improved configuration making it possible to adapt the minute-ventilation measuring circuitry to evaluate intracardiac bio-impedances. It is another object to provide such a measuring circuit that minimizes the additional circuitry required and makes the system simple and advantageous to realize.
More particularly, the invention relates to an improvement of a device such as the one described in the Plicchi U.S. Pat. No. 5,501,702 mentioned above, in which electrodes are placed in a plurality of distinct respective cardiac sites comprising at least one ventricular site and one atrial site, these electrodes being connected to a circuit for the collection of cardiac signals, able to detect a depolarization potential, these electrodes also being connected to a stimulation circuit, able to apply stimulation pulses to at least certain ones of the aforesaid cardiac sites.
This device further includes means for assessing the metabolic needs of the patient by measurement of a trans-pulmonary bio-impedance, these means including a circuit for injecting a current between a first output connected to the case of the device and a second output connected to an injection point located in a first atrial or ventricular site, and a circuit for measuring a differential potential generated by the current injection between a first input connected to the case of the device and a second input connected to a measurement point located in a second atrial or ventricular site, respectively.
The device also includes a means for measuring an intracardiac bio-impedance, including commutation (switch) means that is able to isolate the case from the first output of the current injection circuit and from the first input of differential potential measuring circuit, and to connect the first output of the current injection circuit and the first input of the differential potential measuring circuit to a common reference potential site, i.e., an atrial or ventricular site, distinct from the sites to which are connected the second output of the current injection circuit and the second input of the differential potential measuring circuit, so as to allow a measurement of intracardiac impedance from the signal delivered by the differential potential measurement circuit.
According to the invention, the aforementioned switch means includes means for commuting the circuit element (e.g., an input or an output) to an electric ground potential, operating independently of the current injection circuit and the differential potential measuring circuit.
According to various advantageous subsidiary characterstics of the invention, the switch means are also able to modify the cut-off frequencies of a band pass filter used in the differential potential measuring circuit, preferably in the direction of an increase in the measured frequency band. In one embodiment, the common reference potential site is a left ventricular site, and the points of current injection and differential potential collection are located in distinct respective atrial sites, the measured intracardiac bio-impedance being an atrio-ventricular bio-impedance.
In yet another embodiment, the common reference potential site is a right ventricular site, and the points of current injection and differential potential collection are located in distinct respective atrial sites, the measured intracardiac bio-impedance being a transvalvular bio-impedance.
In still another embodiment, the common reference potential site is a left ventricular site and the points of current injection and differential potential collection are located in distinct respective right ventricular sites, and the measured intracardiac bio-impedance being a inter-ventricular bio-impedance.
In still a different embodiment, the common reference potential site is a right atrial site, and the points of current injection and differential potential collection are located in distinct respective right ventricular sites, and the measured intracardiac bio-impedance is a transvalvular bio-impedance.
In the various embodiments, the aforementioned first and second atrial or ventricular sites are defined by a proximal electrode and a distal electrode, preferably a distal tip electrode of the same atrial or ventricular probe, respectively.